64 Myocardial Metabolism During Increased Work States in the Porcine Left Ventricle In Vivo

It is not known whether myocardial energy requirements can be increased to the degree that they exceed myocardial 02 availability in the absence of abnormalities of coronary blood flow or coronary reserve. To determine whether this form of "demand ischemia" occurs, 10 swine were subjected to pressure overload induced by aortic constriction, inotropic and chronotropic stimnulation by dobutamine, and the combination of these interventions. In an additional 9 animals, intravenous adenosine was administered during the combination of constriction and dobutamine to determine whether further increases in coronary flow could be achieved and if they would attenuate the metabolic changes. Left ventricular anterior wall transmural blood flow was measured by radioactive microspheres. Energy phosphates were assessed by 31P magnetic resonance spectroscopy using the Fourier series window technique to increase the proportion of signal derived from the subendocardium. Myocardial lactate release was quantified independent of net lactate uptake using an isotopic tracer technique. The three interventions produced 39% to 195% increases in myocardial 02 uptake from control measurements. The phosphocreatine to ATP ratio (PCr/ATP), uncorrected for partial saturation, fell significantly, from 1.39±0.10 at control conditions to 1.25+0.10 with dobutamine alone and 1.15±0.08 with dobutamine plus constriction (P<.05 for both). Myocardial lactate release rose from 0.21±0.03 mol *g. min-m at control conditions to 0.45±0.05 and 0.59±0.10 gmol gmin-1, respectively (P<.05 for both), with these two interventions. Although transmurally averaged